Cardiac rhythm management system selecting A-V delay based on interval between atrial depolarization and mitral valve closure

ABSTRACT

A cardiac rhythm management system selects an atrioventricular (A-V) delay based on a time-interval between an atrial depolarization and mitral valve closure (MVC). For several different A-V delays, the system measures time intervals between atrial depolarizations (i.e., sensed or paced P-waves) and accelerometer-detected MVCs. Based on this information, the system selects a particular A-V delay for improving cardiac output during subsequent delivery of cardiac rhythm management therapy.

CROSS-REFERENCE TO RELATED APPLICATION(S)

[0001] This application is a division of U.S. patent application Ser. No. 09/862,763, filed on May 21, 2001, the specification of which is incorporated herein by reference.

TECHNICAL FIELD

[0002] The present system relates generally to cardiac rhythm management systems and particularly, but not by way of limitation, to such a system selecting A-V delay based on interval between an atrial depolarization and a mitral valve closure.

BACKGROUND

[0003] When functioning properly, the human heart maintains its own intrinsic rhythm, and is capable of pumping adequate blood throughout the body's circulatory system. However, some people have irregular cardiac rhythms, referred to as cardiac arrhythmias. Such arrhythmias result in diminished blood circulation. One mode of treating cardiac arrhythmias uses drug therapy. Drugs are often effective at restoring normal heart rhythms. However, drug therapy is not always effective for treating arrhythmias of certain patients. For such patients, an alternative mode of treatment is needed. One such alternative mode of treatment includes the use of a cardiac rhythm management system. Such systems are often implanted in the patient and deliver therapy to the heart.

[0004] Cardiac rhythm management systems include, among other things, pacemakers, also referred to as pacers. Pacers deliver timed sequences of low energy electrical stimuli, called pace pulses, to the heart, such as via an intravascular leadwire or catheter (referred to as a “lead”) having one or more electrodes disposed in or about the heart. Heart contractions are initiated in response to such pace pulses (this is referred to as “capturing” the heart). By properly timing the delivery of pace pulses, the heart can be induced to contract in proper rhythm, greatly improving its efficiency as a pump. Pacers are often used to treat patients with bradyarrhythmias, that is, hearts that beat too slowly, or irregularly. Some pacers coordinate atrial and ventricular contractions to improve pumping efficiency. Cardiac rhythm management systems also include coordination devices for coordinating the contractions of both the right and left sides of the heart for improved pumping efficiency.

[0005] Cardiac rhythm management systems also include defibrillators that are capable of delivering higher energy electrical stimuli to the heart. Such defibrillators also include cardioverters, which synchronize the delivery of such stimuli to sensed intrinsic heart depolarizations. Defibrillators are often used to treat patients with tachyarrhythmias, that is, hearts that beat too quickly. Such too-fast heart rhythms also cause diminished blood circulation because the heart isn't allowed sufficient time to fill with blood before contracting to expel the blood. Such pumping by the heart is inefficient. A defibrillator is capable of delivering a high energy electrical stimulus that is sometimes referred to as a defibrillation countershock, also referred to simply as a “shock.” The countershock interrupts the tachyarrhythmia, allowing the heart to reestablish a normal rhythm for the efficient pumping of blood. In addition to pacers, cardiac rhythm management systems also include, among other things, pacer/defibrillators that combine the functions of pacers and defibrillators, drug delivery devices, and any other implantable or external systems or devices for diagnosing or treating cardiac arrhythmias.

[0006] One problem faced by cardiac rhythm management systems is the proper timing relationship between a sensed or paced atrial depolarization and the subsequent delivery during the same cardiac cycle of a ventricular pacing pulse. This atrioventricular time interval is referred to as the A-V delay. The A-V delay provided by a cardiac rhythm management system may be programmed by the physician to tailor the therapy for a particular patient. The actual value of the A-V delay affects the blood flow from the atrium to the ventricle and, therefore, affects the cardiac output of the heart. The blood flow from the atrium to the ventricle has two components. After the ventricle has completed a contraction, it begins to relax, with blood entering the ventricle from the corresponding atrium when the atrial pressure exceeds the ventricular pressure. This pulse-like fluid flow is sometimes referred to as the “E-wave” of a Doppler echocardiograph. Next, the atrium contracts to actively expel a second pulse-like flow of fluid, sometimes referred to as the Doppler echocardiographic “A-wave,” to the ventricle. For a given fixed time interval between ventricular contractions, if the A-V delay is set too long, then the atrial contraction is moved closer to the preceding ventricular contraction. Because the A-wave and the E-wave occur closer together in time, there is a reduction in total ventricular filling time. By contrast, if the A-V delay is set too short, then the ventricle does not receive the full benefit of the blood flow during the A-wave. For these and other reasons, there is a need to select an A-V delay value that promotes increased blood flow from the atrium to the ventricle, thereby increasing cardiac output.

SUMMARY

[0007] This document discusses a cardiac rhythm management system that, among other things, selects an A-V delay based on an interval between an atrial depolarization and a mitral valve closure.

[0008] In one embodiment, the system includes a method in which ventricular stimulations are provided. The ventricular stimulations are separated from corresponding preceding atrial depolarizations, occurring during the same cardiac cycle, by different atrioventricular (A-V) delays. The system detects mitral valve closures associated with each pair of atrial and ventricular stimulations. The system measures P-MVC time intervals between the atrial depolarizations and the mitral valve closures. In one embodiment, the slopes of the P-MVC time intervals are calculated against the different A-V delays. Based on the slopes, an A-V delay is selected for subsequent delivery of ventricular stimulations. In an alternate embodiment, linear approximations of the P-MVC time intervals (as a function of the different A-V delays) are used for selecting the A-V delay.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] In the drawings, which are not necessarily drawn to scale, like numerals describe substantially similar components throughout the several views. Like numerals having different letter suffixes represent different instances of substantially similar components. The drawings illustrate generally, by way of example, but not by way of limitation, various embodiments discussed in the present document.

[0010]FIG. 1 is a schematic/block diagram illustrating generally, among other things, one embodiment of portions of a cardiac rhythm management system and an environment in which it is used.

[0011]FIG. 2 is a schematic/block diagram illustrating generally one embodiment of portions of a mitral valve closure fiducial point generator.

[0012]FIG. 3 is a graph illustrating an example of different P-MVC time intervals, between atrial depolarizations and corresponding subsequent mitral valve closures, obtained in response to different test A-V delays.

[0013]FIG. 4 is a graph illustrating generally a technique for selecting an A-V delay based on an intersection between a first linear approximation of P-MVC time intervals at short A-V delays and a second linear approximation of P-MVC time intervals at longer A-V delays.

[0014]FIG. 5 is a flow chart illustrating generally one embodiment of an example technique for selecting an appropriate A-V delay at which to deliver subsequent ventricular stimulation therapy.

[0015]FIG. 6 is a schematic/block diagram illustrating generally, among other things, one embodiment of portions of a cardiac rhythm management system including an electrode associated with the left side of the heart, and an environment in which it is used.

DETAILED DESCRIPTION

[0016] In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that the embodiments may be combined, or that other embodiments may be utilized and that structural, logical and electrical changes may be made without departing from the spirit and scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims and their equivalents. In the drawings, like numerals describe substantially similar components throughout the several views. Like numerals having different letter suffixes represent different instances of substantially similar components. The term “and/or” refers to a nonexclusive “or” (i.e., “A and/or B” includes both “A and B” as well as “A or B”).

[0017] This document discusses a cardiac rhythm management system that, among other things, selects an A-V delay based on an interval between an atrial depolarization and a mitral valve closure. The present methods and apparatus will be described in applications involving implantable medical devices including, but not limited to, implantable cardiac rhythm management systems such as pacemakers, cardioverter/defibrillators, pacer/defibrillators, biventricular or other multi-site coordination devices, and drug delivery systems. However, it is understood that the present methods and apparatus may be employed in unimplanted devices, including, but not limited to, external pacemakers, cardioverter/defibrillators, pacer/defibrillators, biventricular or other multi-site coordination devices, monitors, programmers and recorders, whether such devices are used for providing a diagnostic, a therapy, or both a diagnostic and a therapy.

[0018]FIG. 1 is a schematic/block diagram illustrating generally one embodiment of portions of the present cardiac rhythm management system 100 and an environment in which it is used. In this embodiment, system 100 includes, among other things, cardiac rhythm management device 105, which is coupled by leads 110A-B to heart 115. Heart 115 includes four chambers: right atrium 115A, right ventricle 115B, left atrium 115C, and left ventricle 115D.

[0019] In one embodiment, lead 110A includes an electrode associated with right atrium 115A, such as tip electrode 120 and/or ring electrode 125. The electrode is “associated” with the particular heart chamber by inserting it into that heart chamber, or by inserting it into a portion of the heart's vasculature that is close to that heart chamber, or by epicardially placing the electrode outside that heart chamber, or by any other technique of configuring and situating an electrode for sensing signals and/or providing therapy with respect to that heart chamber. Lead 110B, which in one embodiment is introduced into right ventricle 115B, includes an electrode associated with right ventricle 115B, such as electrodes 130 and 135. Device 105 may also include other electrodes, such as housing electrode 140 and/or header electrode 145, which are also associated with heart 115, and which are useful for, among other things, “unipolar” sensing of heart signals or unipolar delivery of contraction-evoking stimulations in conjunction with one or more of the electrodes 120, 125, 130, and 135 associated with heart 115. Alternatively, “bipolar” sensing and/or therapy may be used between electrodes 120 and 125, and between each of electrodes 130 and 135 and another closely situated electrode (not shown).

[0020] In one embodiment, device 105 includes an atrial sensing module 150 and a ventricular sensing module 155, which are each coupled to one or more of the electrodes, such as electrodes 120 and 130, respectively, for sensing intrinsic and/or evoked electrical depolarizations corresponding to heart chamber contractions. Such electrical depolarizations of the heart tissue include atrial depolarizations, referred to as P-waves, and ventricular depolarizations, referred to as QRS complexes. The QRS complex is a rapid sequence of three signal excursions away from a baseline in sequentially switching polarity, with the first excursion referred to as a Q-wave, the second (typically the largest) excursion referred to as an R-wave, and the third excursion referred to as the S-wave. Device 105 also includes atrial stimulation module 160 and ventricular stimulation module 165, respectively coupled, in this example, to atrial electrode 120 and ventricular electrode 130 for providing stimulation energy pulses thereto. Such stimulation energy pulses typically evoke heart contractions of the heart chambers with which their respective electrodes are associated.

[0021] Device 105 also includes a mitral valve closure detector that, in one embodiment, includes accelerometer 170. In one embodiment, accelerometer 170 is carried within the housing of device 105, which is pectorally or abdominally implanted in close enough proximity to heart 115 to sense acceleration from heart 115. Accelerometer 170 outputs a heart acceleration signal to analog-to-digital (A/D) converter 175, for conversion into a digitized signal along with the atrial heart signal output by atrial sensing module 150 and the ventricular heart signal output by ventricular sensing module 155. A/D converter is coupled to controller 180 for providing these digital signals to controller 180.

[0022] Controller 180 includes hardware components and/or microcontroller or microcontroller-like executable operations that implement an accelerometer interface such as mitral valve closure (MVC) fiducial point generator 182, a timer 184, a memory 186, a slope calculation module 188, and an atrioventricular (A-V) delay selection module 190. MVC fiducial point generator 182 is coupled to accelerometer 170 through A/D converter 175 such that it receives a digitized heart acceleration signal. Based upon this digitized heart acceleration signal, MVC fiducial point generator 182 detects mitral valve closures of heart 115 and provides MVC fiducial points associated with the occurrence of such mitral valve closures.

[0023] Timer 184 is coupled to atrial stimulation circuit 160 and/or ventricular stimulation circuit 165 for delivering timing signals that control the delivery of the atrial and/or ventricular stimulation pulses. In an embodiment employing atrial sensing and/or pacing as well as ventricular pacing and/or sensing, these timing signals determine the A-V delay time interval between successive atrial and ventricular senses/stimulations occurring during the same cardiac cycle. Timer 184 also measures the time interval, referred to as a P-MVC time interval, between an atrial contraction (measured either from the issuance of an atrial stimulation pulse or, alternatively, from the detection of a sensed intrinsic or evoked atrial contraction) and a next MVC fiducial point detected by accelerometer 170.

[0024] Timer 184 measures the P-MVC time intervals over several cardiac cycles for which the A-V delay between delivered or sensed atrial and ventricular contractions is varied over a range of values such as, by way of example, but not by way of limitation, approximately between 10 milliseconds and 250 milliseconds, inclusive, at increments that are approximately between 10 milliseconds and 50 milliseconds, inclusive. The measured P-MVC time intervals and corresponding AV delay values are stored in memory locations in memory 186. Based on this data, slope calculation module 188 calculates the slope of the P-MVC time intervals against corresponding adjacent A-V delay values, by taking a difference between adjacent P-MVC time intervals divided by a difference between corresponding adjacent A-V delay values. The resulting calculated slopes are stored in memory locations in memory 186. Based on these calculated slopes, A-V delay selection module 190 determines an appropriate A-V delay for use in subsequent delivery of ventricular stimulations to heart 115 in conjunction with either intrinsic or paced atrial heart depolarizations. In a further embodiment, an indication of the appropriate A-V delay as determined by A-V delay selection module 190 is provided to transceiver 192, which is coupled to controller 180, and transmitted to external interface 194 for display to a physician or other user, such as on a computer monitor, printout, or other data output mechanism.

[0025]FIG. 2 is a schematic/block diagram illustrating generally one embodiment of portions of MVC fiducial point generator 182, including a highpass filter 200, a lowpass filter 202, a highpass filter 205, and a peak detector 210, although it is understood that certain of these components could be combined rather than implemented separately (e.g., a highpass and lowpass filter could be combined into a bandpass filter, etc.). In one embodiment, highpass filter 200 receives the digitized heart acceleration signal from AID converter 175, removes baseline (i.e., constant or low frequency drift) signal components, and provides a resulting output signal to an input of lowpass filter 202. In this example, lowpass filter 202 is a 5-sample moving average “boxcar” filter attenuating signal frequencies above approximately 100 Hz. Lowpass filter 202 receives the baseline-filtered heart acceleration signal from highpass filter 200, and outputs a resulting lowpass filtered heart acceleration signal to an input of highpass filter 205. In one embodiment, highpass filter 205 is a differentiator that takes a first derivative of its input lowpass filtered heart acceleration signal received from the output of lowpass filter 202 and outputs a resulting first derivative heart acceleration signal to an input of peak detector 210. In one embodiment, peak detector 210 detects negative peaks of the first derivative heart acceleration signal. However, it is understood that a polarity reversal of accelerometer 170 and/or signal inversion(s) in the signal processing path of the heart acceleration signal may alternatively require a detection of positive peaks of the first derivative heart acceleration signal. For each cardiac cycle, the first negative peak of the first derivative heart acceleration signal occurring after the delivery of a ventricular stimulation and before the next intrinsic or paced atrial depolarization is deemed an MVC fiducial point associated with the mitral valve closure. An indication of the time at which such MVC fiducial points occur is provided by MVC fiducial point generator 182 to timer 184 for calculation of the corresponding P-MVC time intervals discussed above.

[0026]FIG. 3 is a graph illustrating an example of the different P-MVC time intervals obtained by timer 184 in response to five different A-V delays that were tested, with line segments drawn between the data points. In one embodiment of operation, slope calculation module 188 determines the slopes associated with each P-MVC line segment between corresponding adjacent tested A-V delay values, and stores an indication of such slopes in memory 186. Based on these slopes, A-V delay selection module 190 selects an A-V delay that is deemed appropriate for use in subsequent delivery of ventricular stimulations in conjunction with intrinsic or paced atrial depolarizations. In a very general sense, the shorter pairs of A-V delays typically manifest larger associated slopes than the longer pairs of A-V delays, at least when the extremes of the graph of FIG. 3 are inspected. In one embodiment, the appropriate A-V delay is selected by beginning at the shortest pair of adjacent AV delay values, and working toward the longest pair of adjacent A-V delay values. The appropriate A-V delay is selected as the shortest of the A-V delay pairs with which an adjacent shorter one of the A-V delay pairs provides a larger slope than an adjacent longer one of the A-V delay pairs. This is illustrated in FIG. 3 by the selection of the second shortest A-V delay as the appropriate A-V delay for the timing of subsequent atrial and ventricular senses and/or stimulations, because, proceeding from shorter A-V delays to longer A-V delays, the second line segment manifests a smaller slope than the adjacent first line segment.

[0027] In another embodiment, the “knee” in the hockey stick shaped curve of FIG. 3 is obtained by extrapolating a line based on the shortest A-V delays, and a second line based on the longest A-V delays, determining the intersection of these two lines, and determining the associated A-V delay corresponding to the intersection. This technique, which does not require slope calculations, is illustrated generally by FIG. 4.

[0028]FIG. 5 is a flow chart illustrating generally one embodiment of an example technique for selecting an appropriate A-V delay at which to deliver subsequent ventricular stimulation therapy. At step 500, an test A-V delay is initialized, such as for example, to the lowest A-V delay value in a range of test A-V delay values. At step 505, a ventricular stimulation is delivered after an intrinsic or paced atrial depolarization, separated therefrom by a time equal to the test A-V delay. At step 510, a heart acceleration is sensed. At step 512, the baseline dc or low frequency component of the detected heart acceleration signal is removed by highpass filtering. At step 515, the heart acceleration signal is lowpass filtered. At step 520, the lowpass filtered heart acceleration signal is differentiated to obtain a resulting first derivative heart acceleration signal. At step 525, a first peak of the first derivative heart acceleration signal (i.e., in this case, a first negative peak occurring after the ventricular stimulation and before a next sensed or paced atrial depolarization) is detected and deemed a fiducial point associated with mitral valve closure for that cardiac cycle. At step 530, a P-MVC time interval is measured between the paced or intrinsic atrial depolarization and the corresponding subsequent MVC fiducial point during the same cardiac cycle. At step 535, if the test AV delay is not at the end of the range of test A-V delay values, the test A-V delay is incremented at step 540 and steps 505 through 535 are repeated. If, however, at step 535, the test A-V delay is at the end of the range of test A-V delay values, then at step 545 an appropriate A-V delay for subsequent delivery of ventricular stimulations is selected, such as by using the techniques described with respect to FIG. 3 or 4.

[0029] Although the system and its operation have been particularly described above with respect to selecting an A-V delay for the delivery of right ventricular stimulations (for patients who respond to such therapy), it is understood that the system and its operation is even more applicable to selecting an A-V delay for the delivery of left ventricular stimulations or biventricular stimulations (for patients who respond to such therapy), as illustrated in the schematic/block diagram of FIG. 6. Moreover, the test values of A-V delay and/or the selected A-V delay value may be measured from either right or left atrial depolarizations (paced or sensed). In fact, the data illustrated in FIGS. 3 and 4 is illustrative of data actually obtained using A-V delays taken with respect to the left ventricle.

[0030] In FIG. 6, system 100 further includes a lead 700 having at least one electrode associated with left ventricle 115D, intravascularly, epicardially, or otherwise. In this example, lead 700 is introduced into association with left ventricle 115D by inserting lead 700 through right atrium 115A and into coronary sinus 702 and/or one of its tributaries such as the great cardiac vein. In this manner, an electrode, such as an approximately basal electrode 704 or a more apical electrode 706 is placed in association with a portion of left ventricle 115D for sensing or pacing left ventricular heart contractions. This arrangement also allows delivery of simultaneous or offset biventricular stimulations for coordinating the relative timing contractions of right and left ventricular heart contractions. It further allows the delivery of simultaneous or offset left ventricular stimulations for resynchronizing the spatial nature of the left ventricular depolarization by providing simultaneous or offset stimulations at more than one left ventricular electrode, such as at each of electrodes 704 and 706. The configuration illustrated in FIG. 6 may be used in conjunction with the above-discussed techniques for selecting the appropriate AV-delay, thereby establishing the AV delay value as measured from an atrium to one of electrodes 704 and 706. Such a configuration is particularly useful for, among other things, patients having left bundle branch block (LBBB).

[0031] It is to be understood that the above description is intended to be illustrative, and not restrictive. For example, the above-described embodiments may be used in combination with each other. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” 

What is claimed is:
 1. A method including: providing ventricular stimulations, separated from corresponding preceding atrial depolarizations, occurring during the same cardiac cycle, by different atrioventricular (A-V) delays; detecting mitral valve closures associated with each ventricular stimulation; measuring time intervals between the atrial depolarizations and the mitral valve closures; and selecting, based on the time intervals, an A-V delay for subsequent delivery of ventricular stimulations.
 2. The method of claim 1, in which detecting mitral valve closures includes detecting an acceleration signal that includes information associated with the mitral valve closures.
 3. The method of claim 2, further including highpass filtering the acceleration signal.
 4. The method of claim 3, in which highpass filtering the acceleration signal includes at least one of: (1) removing a baseline component of the acceleration signal; and (2) differentiating the acceleration signal to form a first derivative acceleration signal.
 5. The method of claim 4, further including lowpass filtering the acceleration signal.
 6. The method of claim 4, further including detecting peaks of the first derivative acceleration signal to obtain fiducial points associated with the mitral valve closures.
 7. The method of claim 5, further including detecting negative peaks of the first derivative acceleration signal to obtain fiducial points associated with the mitral valve closure.
 8. The method of claim 5, further including detecting a peak of first derivative acceleration signal that occurs after an R-wave associated with a ventricular contraction and before a P-wave associated with a next atrial contraction to obtain a fiducial point associated with the mitral valve closure associated with said ventricular contraction.
 9. The method of claim 5, further including detecting a negative peak of the lowpass filtered first derivative acceleration signal, wherein the negative peak occurs after an R-wave associated with a ventricular contraction and before a P-wave associated with a next atrial contraction to obtain a fiducial point associated with the mitral valve closure associated with said ventricular contraction.
 10. The method of claim 1, in which the selecting the A-V delay includes: calculating slopes of the time intervals against corresponding A-V delays; and selecting, based on the slopes, the A-V delay for subsequent delivery of ventricular stimulations.
 11. The method of claim 10, in which the calculating slopes of the time intervals includes dividing a difference between the time intervals corresponding to adjacent A-V delay values by a time difference between the adjacent A-V delay values.
 12. The method of claim 10, in which the selecting the A-V delay includes selecting a knee between slopes at small A-V delays and slopes at larger A-V delays.
 13. The method of claim 12, in which selecting the knee includes: forming a first linear approximation of the slopes at small A-V delays; forming a second linear approximation of the slopes at large A-V delays; and finding an intersection between the first and second linear approximations; and selecting an A-V delay associated with the intersection as the A-V delay for subsequent delivery of ventricular stimulations.
 14. The method of claim 10, in which the selecting the A-V delay includes selecting the shortest of the A-V delays with which an adjacent shorter one of the A-V delays provides a larger slope than an adjacent longer one of the A-V delays.
 15. The method of claim 1, further comprising communicating an indication of the selected AV delay from within a body to a location external to the body.
 16. A method including: providing ventricular stimulations separated from corresponding preceding atrial depolarizations by different atrioventricular (A-V) delays; detecting an acceleration signal associated with the heart; differentiating the acceleration signal to form a first derivative acceleration signal; detecting, for each ventricular stimulation, a corresponding mitral valve closure, occurring during the same cardiac cycle as the ventricular stimulation and the preceding atrial depolarization, by detecting a peak of the first derivative acceleration signal, wherein the peak occurs after an R-wave associated with the ventricular stimulation and before a P-wave associated with a next atrial depolarization; measuring P-MVC time intervals between the atrial depolarizations and the corresponding mitral valve closures; calculating slopes of the P-MVC time intervals against the different A-V delays; and selecting, based on the slopes, an A-V delay for subsequent delivery of ventricular stimulations.
 17. The method of claim 16, further comprising communicating an indication of the selected AV delay from within a body to a location external to the body.
 18. A method including: providing ventricular stimulations, separated from preceding atrial depolarizations by different atrioventricular (A-V) delays; detecting a mitral valve closure associated with each ventricular stimulation; measuring P-MVC time intervals between the atrial depolarizations and the mitral valve closures; calculating slopes of the P-MVC time intervals against the different A-V delays; and selecting, for subsequent delivery of ventricular stimulations, the shortest of the A-V delays with which an adjacent shorter one of the A-V delays provides a larger slope than an adjacent longer one of the A-V delays.
 19. The method of claim 18, further comprising communicating an indication of the selected AV delay from within a body to a location external to the body.
 20. A method including: providing ventricular stimulations, separated from preceding atrial depolarizations by different atrioventricular (A-V) delays; detecting a mitral valve closure associated with each ventricular stimulation; measuring P-MVC time intervals between the atrial depolarizations and the mitral valve closures; storing the P-MVC time intervals and the corresponding different A-V delays; forming a first linear approximation of the P-MVC time intervals at small A-V delays; forming a second linear approximation of P-MVC time intervals at large A-V delays, relative to the small A-V delays. finding an intersection between the first and second linear approximations; and selecting an A-V delay associated with the intersection for subsequent delivery of ventricular stimulations.
 21. The method of claim 20, further comprising communicating an indication of the selected AV delay from within a body to a location external to the body. 